Problem: Which of these numbers is composite? ${3,\ 13,\ 53,\ 79,\ 91}$
Answer: A composite number is a number that has more than two factors (including 1 and itself). 3, 13, 53, and 79 each have only two factors. The factors of 91 are 1, 7, 13, and 91. Thus, 91 is the composite number.